In the past several years, tremendous strides forward have been taken in the field of navigational dead reckoning techniques. Among recent developments in this field have been the introduction of computers and the development of velocity, acceleration, and direction sensing devices of high accuracy. However, despite the tremendous advances made in dead reckoning guidance systems employed for bringing a craft or vehicle precisely to a certain geographic location, a fix-taking correctional guidance system must still be used in conjunction with the dead reckoning system because of the characteristic accumulation of dead reckoning error in the latter, if high accuracy of navigation is required.
Generally speaking, the reference data necessary for use in a correctional system can be derived by several techniques and from a variety of sources. Two common methods use celestial observation and the recognition of some earth-fixed parameter. While stellar monitoring can usually be satisfactorily employed at high altitudes, several factors prevent its use in high-speed, low-altitude vehicles. First, weather and cloud cover impose operational limitations in land and air vehicles and in vessels operating at and near the surface of water. Secondly, a turbulent boundary layer is formed during low and medium altitude flights of aerial vehicles which cause image diffusion and defraction and therefore a corresponding degradation in accuracy. Obviously, optical observation of stellar bodies is not readily practicable, in the case of a vessel traveling deep beneath the surface of a body of water, for providing stellar reference data for fix-taking.
One earth fixed parameter data source is topographic information. Many guidance systems were devised in the past which, at least in aircraft, made use of topographic information as reference data for fix-taking. Some of these systems made use of radar derived topographic data, and large efforts were expended in developing radar map matching techniques. Systems of this type have been in existence for some years, but have never been completely satisfactory because of, primarily, their high degree of complexity.
It will be understood that, as employed herein, the term "navigation" refers to the conducting of aircraft and ships from place to place and further is intended to refer, and expressly does refer, to the conducting of any other body from place to place. Thus, while the specific example provided herein is in connection with an aircraft as the vehicle, the sequence of elevations, relative to some fixed reference, from one to the other along a given series of discrete points on the ocean bottom is as unique as along a similar series of points on land, and the elevation sequence along a series of spaced points on land is no less unique when the points are passed over by a land-contacting vehicle than when flown over by an aircraft. The invention, therefore, is specifically applicable also to the navigation of submarine vessels and land vehicles and, in fact, of any body which moves over a surface, the earth's crust being one example thereof, whose altitude varies from place to place with reference to a given altitude datum. While, in the specific example, altimeters are referred to as preferred means for determining both the absolute altitude of an aircraft relative to a reference datum and the height of the aircraft above the earth, the invention is by no means limited to the use of such instruments and its scope is such as to include, in other applications, the use of fathometers and/or pressure-sensing devices giving information indicative of the altitude of the earth's crust and specifically the interval separating a vessel from the ocean bottom and/or surface.
While the term "terrain" ordinarily has been employed, in the past, with references to land areas, it is expressly adopted and employed herein as a term referring to any surface area, such as that of the earth's crust, whether that area be covered with water or air.
Previously proposed fix-taking and navigational systems have sought to utilize terrain elevation data, and they have been based upon the analog comparison of sample data, which are the continuous, analog representation of continuous variations in terrain elevations, with similar data contained in contour maps employed as such. At least some of the sample and known data hence have always been graphically or photographically displayed on actual sheets of paper, rectangles of photographic film, etc., and the values represented thereby have been shown as physically measurable along at least two axes. Because of the nature of the data employed, cumbersome and unwieldy equipments for photographic development, superposition of map over map, orthogonal adjustments of one set of data relative to another, etc. have been unavoidable sources of added weight, complexity, error and malfunction.
One significant improvement of such analog comparison systems is described in U.S. Pat. No. 3,328,795 which does not employ continuously recorded, analog data, but has as one of its bases the use of quantized terrain altitude information taken at discrete points. A numerical comparison of sample and prerecorded data is performed at high speed, and with results predictable and repeatable for the same inputs, by a digital computer. Since the digital computer and associated components are relatively unaffected by noise, vibrations, nuclear radiation, etc., no equipment is required for performing two-dimensional data comparisons, and no feedback or nulling circuitry is needed for determining the point of best physical correlation of the sample with the prerecorded data. As distinguished from systems utilizing analog information, the digital computer is free from the sources of error unavoidably present where analog comparisons are made and hence is not only more accurate but is able to tolerate relatively large errors in sample and known data values without compromising fix-taking accuracy.
Basically the navigation system as described in U.S. Pat. No. 3,328,795 involved the lifting of the contour signature of the prospective navigation region or regions, from available sources such as contour maps or stereo photos. As a vehicle proceeds over the navigation region, a sensor system measures the contour signature of a terrain sample along the vehicle navigation track. As described in U.S. Pat. 3,328,795, the terrain sample is nominally five miles in length to obtain uniqueness and it can be curved or straight. The sample of the measured contour signature from along the navigation track is trial-matched with stored terrain signatures of similar samples from all over the navigation region. The objective of the matching is to determine the location within the navigation region from which the sample of measured navigation track signature was taken. When a best match is determined, the navigation system of U.S. Pat. No. 3,328,795 position fixes within the region and the system updates a dead reckoning navigation subsystem in appropriate coordinates.
An improvement over the navigation system of U.S. Pat. No. 3,328,795 is described in U.S. Pat. No. 4,144,571 and utilizes an update of vehicle position data on a point by point basis. At each terrain measurement, the vehicle position and vehicle velocity data are updated such that the updated data is equal to the previous data, plus the movement of the vehicle due to a measured velocity, plus movement due to a bias in the velocity measurement, plus a characteristic error term. In computing the updated data in U.S. Pat. No. 4,144,571, ground clearance measurements and measurements of absolute altitude above a reference are compared with stored reference data at the predicted vehicle position. As distinguished from a terrain path comparison technique, three vehicle position measurements and three vehicle velocity measurements are updated at each data sample using recursive computation techniques based on past measurement data.
An ideal fix-taking navigation system should possess operational flexibility and should perform satisfactorily where nucler radiation or other adverse environmental and/or flight conditions exist. Moreover, the system should preferably possess the attributes of simplicity accuracy, and reliability and desirably should be compact and light-weight.
A fix-taking navigation system should possess additionally the ability to operate independently of the linearity or nonlinearity of the vehicle path and without previous knowledge of the vehicle movement as this may unexpectedly vary in flight.